1. Field of the Invention
The invention relates to microwave, millimeter wave and submillimeter wave sources and in particular to a pulsed heterodyne transceiver useful for terahertz spectroscopy.
2. Description of the Related Art
Terahertz devices and systems generally refer to creating and detecting electromagnetic energy between 300 GHz and 3 terahertz (3 THz), or wavelengths from 100 to 1000 microns (0.1 to 1.0 millimeters), and also referred to as the submillimeter or far-infrared region of the electromagnetic spectrum. Terahertz energy can be created, for example, using short-pulsed lasers, heterodyne lasers, electronic diode multipliers, free-electron lasers, and BWOs.
One important application of terahertz systems is THz spectroscopy, and more particularly realized as time domain spectroscopy. In such systems, a sequence of femtosecond pulses from a mode locked laser are focused onto suitable semiconductor material to produce THz radiation. The radiation is directed to the target or sample to be analyzed, and a detector or detector array is used to collect the signal propagated through or reflected from the object. Since such measurements are made in the time domain by collecting the timed sequence of pulses, the signals must then be processed by a Fourier transformation to recover the frequency domain spectral information.
Terahertz spectroscopy presents many new instrumentation and measurement applications since certain compounds and objects can be identified and characterized by a frequency-dependent absorption, dispersion, and reflection of terahertz signals which pass through or are reflected from the compound or object. Some current terahertz systems perform analyses in the time-domain by collecting that transmitted signal propagating through the object and then processing the information contained in those signals by a Fourier transformer to produce a spectral analysis. By scanning every point or “pixel” on that object, either on a focal plane or in successive focal planes at different ranges, it is also possible for such a system to perform imaging of the surface or interior cross-sections or layers of the object. This non-invasive imaging technique is capable of differentiating between different materials, chemical compositions, or molecules in the interior of an object. However, the Fourier transform from the time domain into the frequency domain imposes limitations on the frequency resolution and upon the ability to look at specific frequency windows.
As noted in a review article by Peter H. Siegel in, IEEE Transactions on Microwave Theory and Techniques, Vol. 50, No. 3, 915-917 (March 2002), terahertz time-domain spectroscopy was pioneered by Nuss and others at Bell Laboratories in the mid-1990s (B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett., vol. 20, no. 16, pp. 1716-1718, Aug. 15, 1995; D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-ray imaging,” IEEE J. Select. Topics Quantum Electron., vol. 2, pp. 679-692, September 1996), and recently commercialized by at least two companies, Picometrix, LLC of Ann Arbor, Mich. (D. D. Arnone et al., “Applications of terahertz (THz) technology to medical imaging,” in Proc. SPIE Terahertz Spectroscopy Applicat. II, vol. 3823, Munich, Germany, 1999, pp. 209-219) and Teraview Ltd. (a spinoff of Toshiba Research Europe) located in Cambridge, England (D. Arnone, C. Ciesla, and M. Pepper, “Terahertz imaging comes into view,” Phys. World, pp. 35-40, April 2000).
In situ measurements of the transmitted or reflected terahertz energy incident upon a small sample are processed to reveal spectral content (broad signatures only), time of flight data (refractive index determination, amplitude and phase, and sample thickness), and direct signal strength imaging. The principle involves generating and then detecting terahertz electromagnetic transients that are produced in a photoconductor or a crystal by intense femtosecond optical laser pulses. The laser pulses are beam split and synchronized through a scanning optical delay line and made to strike the terahertz generator and detector in known phase coherence. By scanning the delay line and simultaneously gating or sampling the terahertz signals incident on the detector, a time-dependent waveform proportional to the terahertz field amplitude and containing the frequency response of the sample is produced. Scanning either the terahertz generator or the sample itself allows a 2-D image to be built up over time.
Other developments include rapid scanning (S. Hunsche and M. C. Nuss, “Terahertz ‘T-ray’ tomography,” in Proc. SPIE Int. Millimeter Submillimeter Waves Applicat. IV Conf., San Diego, Calif., July 1998, pp. 426-433.) and true 2-D sampling using charge-coupled device (CCD) arrays (Z. Jiang and X.-C. Zhang, “Terahertz imaging via electrooptic effect,” IEEE Trans. Microwave Theory Tech., vol. 47, pp. 2644-2650, December 1999.). In the Picometrix and Lucent Technologies systems, the generator and detector are based on the photoconductive effect in low-temperature-grown (LTG) GaAs or radiation-damaged silicon on sapphire semiconductor. The Teraview system uses terahertz generation by difference frequency mixing in a nonlinear crystal (ZnTe) and detection via the electrooptical Pockels effect (measuring the change in birefringence of ZnTe induced by terahertz fields in the presence of an optical pulse) as first demonstrated by Zhang at the Rensselaer Polytechnic Institute (RPI), Troy, N.Y. (see Q. Wu, T. D. Hewitt, and X.-C. Zhang, “Two-dimensional electro-optic imaging of THz beams,” Appl. Phys. Lett., vol. 69, no. 8, pp. 1026-1028, Aug. 19, 1996.). The femtosecond optical pulses are currently derived from relatively expensive Ti: Sapphire lasers, but other proposals include longer wavelength, especially 1.5 m, solid-state systems that can take better advantage of fiber technology (see D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-ray imaging,” IEEE J. Select. Topics Quantum Electron., vol. 2, pp. 679-692, September 1996). The RF signals produced by the optical pulses typically peak in the 0.5-2 THz range and have average power levels in the microwatt range and peak energies around a femtojoule. This makes T-ray imaging an attractive tool for medical applications (noninvasive sampling), as well as for nondestructive analysis of biological materials or electronic parts. The largest drawback of prior art designs is the need to scan the delay line slowly and over a distance of the desired wavelength resolution (e.g., a 1 GHz resolution would require a 7.5 cm scan) and the inability to look at discrete frequencies of interest.
The need for a multi-octave tunable spectrometer in the THz region is justified by the new suite of applications relating to materials identification facing researchers and system developers today. Historically, the THz field has been dominated by radio astronomers and chemists usually aimed at detecting trace amounts of small gaseous molecules in the interstellar medium or in the Earth's upper atmosphere. The low pressure of the media involved would often lead to narrow, Doppler-limited absorption lines, sometimes less than 1 MHz in linewidth. In roughly the last decade, the THz landscape has changed dramatically with the discovery and demand for detection and imaging of larger molecules, particularly biomolecules and bioparticles. This includes, for example, proteins and vitamins using frequency sweeps above 1 THz, and bacterial spores and nucleic acids using frequency sweeps below 1 THz. In all cases the biomolecular and bioparticle absorption occurs not in the form of narrow lines, but rather as broad “signatures”, typically 1 to 10 GHz or wider. A good example of a bioparticle of current research interest would be the spores of Bacillus subtilis (an Anthrax surrogate), which have recently displayed approximately 6 GHz broad signatures centered around 260 and 420 GHz. In addition, these signatures tend to have less maximum absorption strength than their small molecular counterparts, making them more difficult to “specify” against background noise, standing waves, and other spurious effects. A multi-octave spectrometer capable of measuring small discrete windows of frequencies allows measurement of two or more signatures in the same session, increasing confidence and specificity.
In addition to the time-domain spectrometers noted above, frequency domain systems are also known (See the paper by Verghese et al., “Generation and detection of coherent terahertz waves using two photomixers,” Appl. Phys. Lett., vol. 73, no. 26, pp. 3824-3826, Dec. 28, 1998.). One prior art terahertz spectrometer system is described in U.S. patent application Ser. No. 11/121,350, assigned to the common assignee, and hereby incorporated by reference. The system includes a laser illumination arrangement that generates a pair of source laser beams incident on a source photomixer device or photoconductive switch (PCS) to cause emission of subcentimeter radiation, at least a portion of which interacts with the remote sample to generate a “sample influenced radiation” which is then incident on a detector photomixer device. A second pair of laser beams is incident on the detector to produce an optical component of the detector photocurrent that is offset in frequency with respect to the detected source laser energy. As a result, the detector generates a frequency down-converted electrical output signal responsive to and characteristic of the sample influenced radiation.